1. Field of the Invention
The present invention relates to an apparatus for forming glass, more particularly an apparatus for forming a glass sheet.
2. Technical Background
The manufacture of sheet glass by downwardly flowing molten glass over a forming wedge is well known in the art. However, in practice it was found that the actual width of usable sheet produced was substantially less than the maximum width of usable glass sheet which theoretically could be obtained, as predicated by the longitudinal length of the forming wedge. That is, it has been found that when the molten glass flows along the underside of a negatively inclined surface, such as the downwardly converging surfaces of a forming wedge, the width of the glass stream contracts and pulls inwardly from the vertical ends of the wedge.
This lateral movement of the molten glass produces a bead or thickening portion along the edge of the sheet—the edge head.
The thickened sections on the edges of the sheet not only decrease the amount of usable uniform thickness sheet glass that can be obtained from a given width of drawn sheet, but also limit the speed at which the sheet may be drawn. That is, since the sheet is annealed as a continuous process immediately after formation, and the time required for sheet glass to reach an acceptable strain level in passing through the annealing portion of the process is directly proportional to the thickness of the glass, sheet having thickened edge portions requires more annealing time, thus limiting the maximum rate of sheet formation.
In the past it has been suggested that the narrowing of the width of glass sheet formed by the downdraw process may be prevented by utilizing rollers or edge cooling.
However, this has not been entirely satisfactory since rolling merely attempts to reduce the thickness of the beaded portion without attempting to correct the cause or source of the bead, and cooling tends to enlarge the formation of beaded edges, which as previously indicated are detrimental in the formation of sheet glass. Further, high viscosities adjacent the edges of the sheet flow actually tend to create a pull-in or narrowing of the glass to a much greater extent than do low viscosities. However, should the edges be chilled sufficiently to set up the glass, additional detrimental effects are produced in the form of sheet warpage and undesirable stresses.
Prior art methods of increasing sheet width have employed a web surface portion extending between the downwardly converging surfaces of the forming wedge and a projecting edge surface portion which aided in increasing the width of the drawn glass sheet. U.S. Pat. No. 3,451,798 discloses such a web surface portion which terminates at its lowest extent at the horizontal plane passing through the root, the line along which the downwardly converging surfaces of the forming wedge meet. U.S. Pat. No. 3,537,834 discloses a forming apparatus comprising a web surface portion which, at its lowest point may be extended below the root.
In one prior art apparatus, illustrated in FIG. 1, an overflow trough 10 comprising converging forming surfaces includes multiple edge directors 12. Each edge director 12 comprises two main portions: a projecting edge surface portion 14 which intersects forming surface portions of the trough along its vertical extent, and a web or filleted surface portion 16 which extends between the projecting edge surface portion 14 and one of the downwardly inclined converging surface portions. However, web surface portion 16 does not extend below the lower apex formed by the converging forming surfaces.
In another prior art apparatus, shown in FIG. 2, web surface portion 20 extends below the lower apex, or root, as indicated by point 22. However, whether web surface portion 20 is planar, curvilinear or frusto-conical in shape, the extended web surface portion does not break or kink (i.e. demonstrate an abrupt change in direction) along its downward length.
Although the prior art devices described above have been useful to extend the width of glass sheet drawn from overflow troughs, devices capable of still further improvement in sheet width are needed. Unfortunately, there are practical limits to the length of the forming wedge without incurring problems associated with sagging of the wedge. Thus, there is a need to further increase the width of glass sheet drawn from a forming wedge which does not entail increasing the length of the forming wedge itself.
The present invention virtually obviates the problems of reduction in sheet width and bead formation heretofore encountered when forming sheet glass by an overflow downdraw process by providing the forming wedge with edge director projections having web portions and extension portions which intersect with the web portions.
Each edge director has a projecting edge surface portion which extends along edge portions of the forming surface of the wedge, and a web or filleted portion which extends between such projecting edge portion and an adjacent downwardly inclined forming surface portion.
The web or fillet portion and the extension surface portion of each edge director provides a wetted length in the horizontal direction which is greater than the horizontal length of the negative-angle wedge surface which it intercepts, and accordingly spreads out the flowing glass and decreases its thickness adjacent to its ends before the glass is drawn off an edge of the extension surface portion to inhibit bead formation.
In an embodiment of the invention, an apparatus for drawing sheet glass is disclosed which includes a forming wedge having a pair of downwardly inclined forming surface portions. The downwardly inclined forming surface portions converge at the bottom of the forming wedge to form a root and define a glass draw line therealong. A web surface portion for intercepting and thinning the flow of glass along edge portions of the forming surfaces intersects at least one of the forming surfaces. An extension surface portion intersects with and extends below the web surface portion, and an outwardly directed normal to the extension surface portion has a downwardly directed component (i.e. vector component). Preferably, the first extension surface portion is detachably cooperative with the first web surface portion. That is, the first extension surface portion can be removed. The first extension surface portion may be attached to the first web surface portion via one or more dovetail joints, for example, thereby facilitating detachment of the first extension surface portion from the web surface portion. Alternatively, pins may be embedded within the first extension surface portion which are sized to cooperate with corresponding receiving orifices in the web surface portion. Beneficially, the first extension surface portion may be removed my glass is flowing over the forming surfaces, thereby minimizing down time of the glass sheet forming operation.
Preferably, the extension surface normal forms an angle α with the web surface portion normal in a vertical plane through the web portion surface and the extension surface portion greater than about 15° and less than about 30°.
An inner edge of the extension surface portion preferably intersects a vertical plane intersecting the root.
In another embodiment an apparatus for drawing sheet glass is provided including a forming wedge having a pair of downwardly inclined forming surface portions, the downwardly inclined forming surface portions converging at the bottom of the forming wedge forming a root and defining a glass draw line therealong. An edge director extends along vertical edge portions of the forming surfaces, the edge director for intercepting and thinning the flow of glass along edge portions of the forming surfaces having a web surface portion which intersects with the forming surfaces. An extension surface portion intersects with and extends below the web surface portion. An outwardly directed normal to the extension surface portion has a downwardly directed (vector) component.
Preferably, the first extension surface portion is detachably cooperative with the first web surface portion. The first extension surface portion may be attached to the first web surface portion via one or more dovetail joints, for example. Alternatively, pins may be embedded within the first extension surface portion which are sized to cooperate with corresponding receiving orifices in the web surface portion.
Preferably, the normal to the web surface portion and the normal to the extension surface portion form an angle β in a vertical plane passing through both the web surface portion and the extension surface portion between 15° and 30°.
In still another embodiment of the invention, a method of making a glass sheet is provided comprising flowing molten glass over a pair of downwardly inclined forming surface portions comprising a forming wedge, the downwardly inclined forming surface portions converging at the bottom of the forming wedge, flowing the glass sheet over a web surface portion intersecting with at least one of the forming surface portions, and flowing the glass sheet over an extension surface portion intersecting the web surface portion, the web surface portion having an outwardly directed surface normal with a downwardly directed (vector) component.
The invention will be understood more easily and other objects, characteristics, details and advantages thereof will become more clearly apparent in the course of the following explanatory description, which is given, without in any way implying a limitation, with reference to the attached Figures. It is intended that all such additional systems, methods features and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.